The invention is particularly applicable in the oil industry for transferring personnel from an off-shore platform to a boat and vice versa.
The problem arises because these platforms are sometimes located far out to sea where the operation of transferring personnel is made dangerous by the swell.
French patent application No. 2493 290 describes a device for transporting personnel by means of a crane between a first support on which the crane is disposed and a second support situated at a lower level than the first support and subject to relative movement relative to the first support. This device includes a nacelle having two separate winches enabling one of them to unwind a cable upwardly while the other unwinds a cable downwardly.
The two winches are driven by a single constant couple motor via a differential planetary reduction gear. A ratchet system enables rotation in one direction on one or other of the winches to be blocked on command. When the ratchet on one of the winches is not in the blocking position, the corresponding winch can rotate in both directions which enables it to wind in and out in time with the swell.
When the load is suspended in the air, the ratchets on the upper winch bear the entire load (the nacelle).
Although this system is satisfactory, it suffers from certain drawbacks. It is rather complicated to control and requires the crane operator to perform actions to which he is not accustomed. It includes a feed cable which is connected to the crane and which connects the nacelle to the crane. The ratchet system requires safety devices. Finally, although a load can be lifted automatically and without shocks from a boat by means of the heaving compensation device, the same is not true when a load is lowered onto the deck of a boat in which case the softness of the landing depends on the skill of the crane driver in allowing for the swell. In fact, the lowering system does not compensate for heaving but only ensures lateral guidance of the nacelle, which in itself is a most important function.
Belgian Pat. No. 864 814 describes a device for stably suspending a load beneath a boat, however in this device said boat necessarily includes special arrangements which firmly link the boat to said load and which prevents the load, for example a diving bell, from being suspended from any boat.